Inorganic building materials such as concrete, mortar and slate boards or tiles have fragile surface layers and it is common practice to prime their surfaces for surface reinforcing and improving their bonding and adhesive affinities for finish coatings. As a primer for such purposes, a primer composition comprising a synthetic polymer dissolved in an organic solvent has been in broad use. However, containing an organic solvent which is volatile, any primer of this type is not acceptable from safety and hygienic points of view, for the solvent vapor is liable to catch fire and on inhalation exerts ill effects on human health. Under the circumstances, the present applicant previously developed a water-based or aqueous primer not containing an organic solvent and filed a patent application (JP Application S-62-80694). Following the development of that primer, as an improved version thereof, the applicant further developed a one-part type aqueous primer composition, which is obtainable by copolymerizing a dimethylaminoalkylacrylamide or dimethylaminoalkylmethacrylamide-epichlorohydrin adduct with an alkoxysilane and which provides an aqueous resin system exerting a powerful reinforcing effect on the substrate surface layer on crosslinking regardless of the alkalinity of the substrate, and filed a patent application (JP Application H-3-229766).
However, the crosslinkable monomer used as a comonomer unit of the above aqueous primer is a per se hydrophilic monomer and since some of the functional groups remain uncrosslinked and hydrophilic in the dried film, the primer is not as satisfactory as a solvent-based primer in the water resistance and water tightness of the film.